The experiments proposed here are based on and will extend the recent in vitro experiments by Disterhoft and collaborators. They have demonstrated conditioning-specific biophysical alterations in hippocampal CA1 pyramidal neurons of brain slices from nictitating membrane/eye retraction conditioned rabbits. Rabbits will be conditioned in a tone discrimination reversal nictitating membrane paradigm. Hippocampectomized rabbits cannot learn this discrimination reversal successfully. Pseudoconditioned, naive and antidromic activation rabbits will be controls. Extracellular recording from CA1 and CA3 pyramidal cells will be done to determine if hippocampal engagement is unilateral when monaural tone CS's are paired with an ipsilateral puff US. If hippocampal changes are unilateral, one hippocampus can serve as a within-animal control in the brain slice experiments. In vitro hippocampal slices will be prepared and maintained with standard procedures. The experiments will be carried out "blind", without experimenter knowledge of the behavioral training history of the rabbits. Biophysical parameters to be measured include spike amplitude, resting potential, input resistance, sag, AHP magnitude and duration. A current, accommodation, EPSP and IPSP amplitude. The calcium-mediated potassium current, which underlies the slow AHP, and the fast, voltage activated potassium current, IA, will receive particular attention. Pyramidal neurons and the interneurons intermixed with them will be studied to determine if biophysical alterations are found in both cell types after conditioning. CA1 and CA3 regions will be examined to determine if ionic alterations in these functionally connected regions are interdependent. Stages of discrimination reversal acquisition will be varied to determine when biophysical alterations become established during learning. The possibility that cellular changes become consolidated between training sessions will be evaluated by studying slices made immediately after or 24 hours after the training session. Evidence for a correlation between biophysical and behavioral measures across the total learning curve will be sought. The research program will begin probing the cellular substrates of mammalian associative learning in a novel way by using the in vitro brain slice. This approach allows a biophysical analysis of learned alterations in specified cell population not previously possible.